This invention relates to a circuit arrangement for use in a control station of a radio communication system and, in particular, to a circuit arrangement for a control station of a land mobile telephone system.
As will later be described with reference to one of several figures of the accompanying drawing, a land mobile telephone system generally comprises a control station comprising, in turn, an exchange and a plurality of base stations, each having a radio or service zone. The base stations are fixedly located at geographically different sites so that the respective service zones may cover a service area. Alternatively, each of the service zones may be geographically independent of the others. A plurality of land mobiles, namely, automobiles may be present in a service zone of a base station accompanied by the exchange. The automobiles thus serve as substations comprised by the land mobile telephone system. Some of the substations are communicable with the other subscribers through the base station under consideration. The substations are also communicable with other substations in another service zone through the control station. Depending on the circumstances, no substation may be present in the service area.
A plurality of speech channels and a few control channels are allotted to each zone. The control channel comprises a paging channel for selectively calling up automobiles in the service zones. The paging channel is common to a plurality of the service zones. Therefore, the whole of the system is adversely affected when the paging channel is locally interrupted or suspended by any jamming or interfering waves.
This means that occurrence of jamming waves should be monitored and detected to favorably operate the control system without any interruption.
In order to detect the jamming waves, a conventional system has measured an intensity of electric field during an idle or unused period, such as at night or the like, of the system. However, it is impossible for this conventional system to find out temporary jamming waves and to rapidly recover the system from interruption.
In another conventional control system, the intensity of electric field of a test signal is periodically measured during operation of the system. With this system, communication should, however, be periodically interrupted intermittently for the purpose of measuring the field intensity.
In both of the conventional systems, no automobile is communicable with a base station in the service area on occurrence of jamming waves for a long time.